This invention relates to a transmission structure for an agricultural tractor having a double clutch mechanism consisting of a propelling system clutch and a power takeoff system clutch operable in sequence, an inner and outer double shaft construction extending from the double clutch mechanism and consisting of a propelling system drive shaft and a power takeoff system drive shaft, a propelling system gear transmission housed in the gear transmission case and adapted to receive an output of the propelling system drive shaft, and a power takeoff system gear transmission housed in the gear transmission case and adapted to receive an output of the power takeoff system drive shaft.
This type of transmission structure is disclosed, for example, in a Japanese utility model application laid open or published for public inspection under No. 56-42429. In the disclosed arrangement, a tractor body comprises a gear transmission case formed of cast iron and connected to an engine, and the gear transmission case contains a double clutch and an outer and inner double drive shaft construction.
With the known arrangement noted above, the gear transmission case must have a considerable strength in order that the propelling system drive shaft and power takeoff system drive shaft in the double shaft construction contained in the gear transmission case will not be affected by a distortion of the gear transmission case due to a tractor weight. The double clutch is incorporated only into a relatively large tractor having an engine and a gear transmission case directly interconnected.
In recent years, however, it is desired to incorporate a double clutch also into a small lightweight tractor often including as part of its body a housing formed of sheet metal. With this type of small tractor, it is inevitable that the tractor body including the housing formed of sheet metal becomes distorted under a tractor weight or other loads. It is therefore practically impossible to incorporate a double clutch into a small tractor as such unless the propellng system and power takeoff system drive mechanisms are positively protected against the distortions of the tractor body.